1. Field of the Invention
The present invention relates to telephone paystations and more particularly to a transmitter ground detect circuitry to prevent users from fraudulent use of the paystation instrument by grounding the transmitter.
2. Description of the Related Art
Telephone paystations are frequently designed to work in the emergency prepay mode. That is to say they do not require a coin to be deposited to enable the dial or associated telephone keypad (utilized where tone signaling is incorporated in the paystation). This arrangement is provided so that telephone users may call emergency numbers (911, etc.) without making the necessary coin deposits. Should the user dial a number other than an emergency number as designated by the telephone central office, the central office will perform a coin test to determine if the initial rate or amount of coins required have been properly deposited.
This coin test looks for the presence of ground current or a difference in current through the two loop conductors extending between the telephone central office and the paystation. When the initial rate (10 cents, 15 cents, 25 cents, etc.) has been deposited, the paystation then connects a load, in most instances a coin relay circuit to an earth ground. This load then provides the necessary current path back to the telephone central office to indicate that initial coin deposit has been made. In the case of many telephone central offices this same condition can be simulated by grounding the transmitter included in the telephone handset.